Overview of Bone Grafts
The rapid and effective repair of bone defects caused by injury, disease, wounds, or surgery has long been a goal of orthopaedic surgery. Toward this end, a number of compositions and materials have been used or proposed for use in the repair of bone defects. The biological, physical, and mechanical properties of the compositions and materials are among the major factors influencing their suitability and performance in various orthopaedic applications.
Much effort has been invested in the identification and development of alternative bone graft materials. Urist has published seminal articles on the theory of bone induction and a method for decalcifying bone, i.e., making demineralized bone matrix (DBM). Urist M. R., Bone Formation by Autoinduction, Science 1965; 150(698):893-9; Urist M. R. et al., The Bone Induction Principle, Clin. Orthop. Rel. Res. 53:243-283, 1967. DBM is an osteoinductive material, in that it induces bone growth when implanted in an ectopic site of a rodent, owing to the osteoinductive factors contained within the DBM. Honsawek et al. (2000).
DBM implants have been reported to be particularly useful (see, for example, U.S. Pat. Nos. 4,394,370, 4,440,750, 4,485,097, 4,678,470, and 4,743,259; Mulliken et al., Calcif Tissue Int. 33:71, 1981; Neigel et al., Opthal. Plast. Reconstr. Surg. 12:108, 1996; Whiteman et al., J. Hand. Surg. 18B:487, 1993; Xiaobo et al., Clin. Orthop. 293:360, 1993, each of which is incorporated herein by reference). DBM typically is derived from cadavers. The bone is removed aseptically and treated to kill any infectious agents. The bone is particulated by milling or grinding, and then the mineral component is extracted by various methods, such as by soaking the bone in an acidic solution. The remaining matrix is malleable and can be further processed and/or formed and shaped for implantation into a particular site in the recipient. Demineralized bone prepared in this manner contains a variety of components including proteins, glycoproteins, growth factors, and proteoglycans. Following implantation, the presence of DBM induces cellular recruitment to the site of injury. The recruited cells may eventually differentiate into bone forming cells. Such recruitment of cells leads to an increase in the rate of wound healing and, therefore, to faster recovery for the patient.
Overview of Total Hip Joint Replacement Arthroplasty
Total hip joint replacement arthroplasty can provide a patient with dramatically improved quality of life by relieving pain and offering increased mobility. Total hip joint replacement arthroplasty is a surgical procedure wherein diseased portions of the hip joint are removed and replaced with artificial prostheses, such as a femoral component and an acetabular cup. The acetabular cup is fitted in or against the acetabulum. It is noted that the acetabulum comprises the ilium, the ischium, and the pubis. These bones are fused at the acetabulum. For ease of reference, the acetabulum will be discussed as a single structure. Successful replacement of deteriorated, arthritic, or severely injured hips has contributed to enhanced mobility and comfortable, independent living for many people who would otherwise be substantially disabled.
There generally are two broad classes of joint arthroplasty procedures: primary joint replacement arthroplasty and revision arthroplasty. Primary joint replacement is when the original, biological joint is removed and replaced with an implant. Revision arthroplasty is when the primary joint replacement fails and must be replaced.
A failed prosthesis and/or dislocation of a total hip replacement generally causes pain, reduces the ability to work, and necessitates a revision operation. Prosthesis failure and/or dislocations can result from a variety of causes, such as soft tissue laxity, loosening of the implant, and impingement of the femoral neck with either the rim of an acetabular cup implant or the soft tissue or bone surrounding the implant. Loosening of the implant is often due to bone loss around the implant, caused by adverse tissue reactions to wear particles.
Revision arthroplasty involves additional challenges over primary joint replacement because, in addition to placement of the revision implant, the primary implant must be removed. A common problem with revision arthroplasty is a loss of bone stock associated with the removal of bone cement, or osteolysis due to wear debris and the body's reaction to it, or from stress shielding, or a combination of these. Further, in some instances, upon insertion into the acetabulum of an implant, voids may remain between the back surface of the implant and the pelvic bone remaining in the acetabulum. In cases where there is a defect in the area of the acetabulum, or behind the acetabulum, the surgeon will often wish to fill the defect in some way. Bone graft material is sometimes applied to the acetabulum to encourage bone growth between the acetabulum and the acetabular cup. Frequently, the bone graft material falls through voids in the acetabulum.
Commonly, the acetabular cup prosthesis is manufactured of a polymeric material, such as polyethylene. A backing is commonly placed between the acetabular cup prosthesis and the acetabulum. In the past, metal backings have been widely used, at least in part because a stiff backing was believed to be mechanically favorable. It has more recently been determined that a stiff backing causes two problems: It generates higher stress peaks around the acetabular rim than those caused by full polyethylene cups, and it reduces the stresses transferred to the dome of the acetabulum, causing stress shielding.
It would be useful to provide a backing for an acetabular cup prosthesis using bone graft materials such that the prosthesis aids in holding graft material in place, encourages bone apposition up to the implant or, in the case of an implant with a porous metallic coating, encourages ingrowth and biological attachment to the implant.